The Oscar-nominated actress, 32, plays Glinda, the Good Witch of the North in the big-budget film inspired by L. Frank Baum's beloved Oz books (which inspired the 1939 classic film The Wizard of Oz). For the premiere, Williams, at first glance, appeared to showcase her signature style: With her now-familiar blonde pixie cut, Williams wore a nearly floor-length, short-sleeved white gown with gold-and-silver floral details by Prada, plus Prada shoes and a Kate Young for Target clutch.

From the side, however, Williams' look wasn't so demure -- featuring a slit that went above her waist to reveal her black underwear beneath. "It wasn't intentional! It was just a good fit!" Williams told PopSugar of the frock.

Dating Jason Segel for about two years now, Williams is single mom to Matilda, her 6-year-old daughter with the late Heath Ledger. Williams noted she's excited for Matilda to see her in Oz -- and told PopSugar she's planning on screening the fantasy flick for Matilda on a less "overwhelming" night than Wednesday's big premiere.